Multilevel pulse position modulation for efficient fiber optic communication

ABSTRACT

Decreasing the average transmitted power in an optical fiber communication channel using multilevel amplitude modulation in conjunction with Pulse Position Modulation (PPM). This multilevel PPM method does not entail any tradeoff between decreased power per channel and channel bandwidth, enabling a lower average transmitted power compared to On/Off Keying (OOK) with no reduction in aggregate data rate. Therefore, multilevel PPM can be used in high-speed Dense Wavelength Division Multiplexed (DWDM) systems where the maximum number of channels is traditionally limited by nonlinear effects such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS). This modulation technique can enable an increased number of channels in DWDM systems, thereby increasing aggregate data rates within those systems.

PRIORITY AND RELATED APPLICATIONS

[0001] The present application claims priority to provisional patentapplication entitled, “Multilevel Pulse Position Modulation ForEfficient Fiber-Optic Communication,” filed on Mar. 29, 2001 andassigned U.S. Patent Application Serial No. 60/279,655.

FIELD OF THE INVENTION

[0002] The present invention relates to optical fiber communicationsystems and more particularly relates to decreasing the transmittedpower of signal transmission over an optical fiber communicationchannel, while maintaining channel bandwidth, through the use ofmultilevel pulse position modulation.

BACKGROUND OF THE INVENTION

[0003] In virtually all fields of communications, there exists apersistent demand to transmit more data in less time. The amount ofinformation that can be transmitted over a communications system (orthrough a component of that system) is referred to as the bit rate orthe data throughput of the system. Traditionally, system throughput isincreased by either increasing the number of channels carryinginformation or increasing the bit rate of each channel. In order to meetever-increasing bandwidth demands, aggregate throughput in fiber optictransmission systems has conventionally been increased by using multipleWavelength Division Multiplexed (WDM) channels,time-division-multiplexing (TDM), or some combination of the twotechniques. WDM techniques increase the number of channels transmittedon a particular fiber, while TDM techniques increase the data rate ofeach individual channel.

[0004] Conventional optical fiber networks typically can deliver on theorder of 10 Gigabits of data per second (10 Gb/s). Both WDM and TDMtechniques have been applied to realize fiber channel bit rates wellabove this conventional 10 Gb/s capacity. Many fiber optic communicationsystems comprise multiple WDM channels simultaneously transmittedthrough a single optical fiber. Each of these channels operatesindependently at a given bit rate, B. Thus for an m channel WDM system,the system throughput is equal to m×B. Conventional Dense WDM (DWDM)systems typically operate with 40 to 100 channels. There are certainrestrictions, however, that limit the aggregate power that can betransmitted through a single DWDM optical fiber (i.e., the launchpower). For example, eye safety power regulations and nonlinear effectsin the fiber place limits on the aggregate launch power. In addition,channel spacing limitations and per-channel launch power, effectivelylimit the number of WDM channels that can be combined for transmissionon a single fiber.

[0005] Optical fiber networks are typically comprised of a series oflinks that include a transmission block, a receiver block, and a longstretch of optical fiber connecting the two blocks (i.e., the opticalplant). FIG. 1 is a block diagram of a conventional m-channel WDM fiberoptic transmission system link 100. The fiber optic transmission systemlink 100 consists of a WDM transmission block 102 (denoted as the“Head”), the optical fiber 104, and a WDM reception block 106 (denotedas the “Terminal”). The Head 102 comprises m transmitters 108-112(labeled “Tx”) and an m-channel WDM multiplexer 114. Each transmitter108-112 comprises an optical source (not shown) and all circuitrynecessary to modulate the source with the incoming data stream. For thecase of external modulation, the transmitter block also includes amodulator. The Terminal 106 comprises an m-channel WDM demultiplexer 116and m receivers 118-122 (labeled “Rx”). Each receiver 118-122 comprisesa photodetector (not shown) and all circuitry required to operate thedetector and amplify the detected signal in order to output the originalelectrical data stream.

[0006] For 10 Gb/s transmission in optical fiber, chromatic dispersioncan present a potentially significant transmission problem. Anytransmitted optical signal will have a spectral width associated withit. As data rates increase for on-off key modulated signals, thespectral width of the modulated signal increases as well. Because therefractive index of a fiber medium, such as silica fiber is a functionof wavelength, different components in the spectrum of the opticalsignal will travel at different velocities through the fiber. Thisphenomenon is known as chromatic dispersion, and it can present asignificant source of distortion and inter-symbol interference (ISI) forhigh-speed optical transmission over long lengths of fiber. Conventional10 Gb/s links of 75 kilometers or longer typically utilize some type ofdispersion compensation to offset this effect. Such dispersioncompensation is typically implemented in the form of dispersion-shiftedfiber (DSF) that counteracts the dispersive effects of standard fiber.

[0007] In order to upgrade existing fiber optic transmission systems for10 Gb/s signaling, dispersion compensation can become an even morecomplex issue. In order to realize channel data rates of 10 Gb/s andbeyond, the optical fiber 104 as well as the Head 102 and Terminal 106of the link 100 are typically upgraded to support the increased datarates. In order to increase the channel bit rates in this conventionallink 100, each transmission block 102 and reception block 106 must bereplaced with optical components and circuitry capable of achieving thedesired bandwidths. For high-speed channel bit rates (10 Gb/s andfaster), the optical fiber 104 also must often be replaced in order tocompensate for signal distortions, which are more prominent at higherdata rates. This process can be particularly cumbersome and costly in along-haul link where hundreds of kilometers of fiber must be replaced.For existing long-haul optical links, the complexity and cost ofreplacing planted fiber often represents a prohibitive barrier forincreasing channel bit rates.

[0008] Service providers seeking to optimize revenue and contain costprefer a highly granular, incremental expansion capability that is costeffective while retaining network scalability. The ability to increasethe throughput capacity of single point-to-point links or multi-spanlinks without upgrading or otherwise impacting the remainder of thenetwork is highly desirable from an engineering, administrative andprofitability standpoint. It is also desirable to decrease the powerrequired to transmit a signal over an optical fiber communicationsystem. However, power efficiency cannot normally be realized at thecost of data throughput rates.

[0009] Dense wavelength division multiplexing (DWDM) technologycurrently enables high aggregate data rates in long-haul fiber optictransmission systems. The maximum power per WDM channel on a singlefiber link is limited by several well-known nonlinear effects includingself-phase modulation (SPM), cross-phase modulation (XPM), four-wavemixing (FWM), stimulated Brillouin scattering (SBS), and stimulatedRaman scattering (SRS). Since a given fiber optic system will haveinherent limits on the maximum total power that can be transmitted on asingle fiber, these nonlinear effects ultimately limit the maximumnumber of channels, i.e., wavelengths, in a DWDM system. For many WDMsystems, particularly long-haul transmission links, it is desirable toincrease the number of WDM channels, thereby increasing the totalaggregate data rate of the system.

[0010] In order to meet growing demands for higher data throughput inWDM fiber optic transmission systems, more channels per fiber aredesired. The detrimental effects (such as channel cross-talk andsignal-to-noise degradation) due to nonlinear interactions such as FWMincrease as channel spacings decrease. Accordingly, simply narrowing theWDM channel spacing is not a completely satisfactory solution. However,because decreasing the transmitted power per channel can reduce manynonlinear effects in the system, one solution entails simultaneouslyreducing the power per channel and the channel spacing to realize agreater number of channels. Advantageously, decreasing the power perchannel while maintaining the channel spacing can increase thetransmission length of a given WDM system.

[0011] Compared to on-off keying (OOK), alternative modulationtechniques such as pulse position modulation (PPM) can reduce thetransmitted power per channel. However, in the specific case of PPM, theincreased efficiency can be realized at the cost of decreased bandwidth.Using this method of modulation, a transmitted symbol, or cell, isdivided into a discrete number of equally spaced temporal positions. Onepulse, or chip, is transmitted per cell, occupying one and only one ofthe temporal positions within that cell. In this way, data is encodedinto the temporal position of a chip within its particular cell.

[0012] As an example of the PPM format, FIG. 3 illustrates an 8-PPM(eight temporal positions per cell) data stream with a cell period of Tand a chip duration of τ, which is one eighth the cell period. This8-PPM modulation format could be used to multiplex three independent OOKdata streams (each with bit rates equal to T⁻¹) since there are eight(2³) chip positions available in each cell. Assuming that each of thethree multiplexed channels consists of a data stream with an equalpercentage of Os and Is transmitted, the 8-PPM data stream as shown inFIG. 3 can operate at {fraction (1/12)} the average transmitted power ofthe three OOK channels combined.

[0013] Although PPM requires less average transmitted power thanconventional OOK, overall bandwidth in the link is decreased. In orderto multiplex n OOK channels (each with a bit rate of T⁻¹) in n-PPMformat for optical transmission, the link would require electronics andoptical components that could operate with bit rates of 2^(n)/T. If weconsider the example of 8-PPM shown in FIG. 3, the components requiredto transmit such a signal would need to be capable of operating at a bitrate of 8/T in order to transmit chips of duration τ=T/8. However, theaggregate data rate of the 8-PPM system would be 3/T (number ofchannels·OOK channel bit rate). In general, in order to transmit ann-PPM data stream, components with a data rate of 2^(n)/T are required,but the aggregate data rate of the system would only be n/T. Forhigh-speed fiber optic links, greater bandwidth is typically preferredover low power transmission, making PPM a less desirable solution forthese applications.

[0014] PPM may also be used to reduce the average transmitted power on asingle channel. For example, n consecutive bits in a OOK data stream(with a bit rate of B) may be encoded into a 2^(n)-PPM signal with acell period of n/B. In this case, the 2^(n)-PPM signal would transmit1/2^(n−1) the average transmitted power of the OOK data stream. However,the 2^(n)-PPM signal would require components with data rates up to2^(n)-B/n to maintain the data rate of the incoming OOK signal. In otherwords, the transmitting and receiving components in the link mustoperate at data rates that are faster than the original data rate, B. Asin the previous case, a trade-off exists between average transmittedpower and the bandwidth of the components.

[0015] PPM has been used in free-space data transmission systems and haseven been demonstrated for fiber optic transmission. Although PPMenables lower average transmission powers, the corresponding tradeoffwith channel bandwidth has prevented its commercial implementation inconventional fiber optic systems, particularly long haul DWDM systems.

[0016] In view of the foregoing, there is a need to implement PPM in thecontext of a fiber optic communication system to reduce the requiredtransmitted power. However, the use of PPM in the fiber opticscommunication system should not reduce the system throughput (i.e.,bandwidth). The present invention combines multilevel amplitudemodulation with PPM to achieve efficient optical data transmissionwithout a subsequent decrease in channel bandwidth. Moreover, the PPMimplementation should not require replacing an existing optical fiberplant or necessitate a change in the expensive optical components.

SUMMARY OF THE INVENTION

[0017] The present invention can be used to decrease the averagetransmitted power per Wavelength Division Multiplexed (WDM channel)using multilevel amplitude modulation in conjunction with pulse positionmodulation (PPM). The multilevel PPM method of the present inventiondoes not entail any tradeoff between decreased power per channel andchannel bandwidth, allowing for a lower average transmitted powercompared to OOK with no reduction in aggregate data rate. Therefore,multilevel PPM is applicable to high-speed DWDM systems where themaximum number of channels is currently limited by nonlinear effects inthe fiber. This modulation technique can enable an increased number ofchannels in DWDM systems, thereby increasing aggregate data rates withinthose systems.

[0018] The present invention enables the combination of N data streamsinto one m-level amplitude modulated n-PPM (n available temporalpositions within each transmitted cell) data stream, where m·n=2^(N).The m levels of amplitude modulation combined with the n chip positionswithin each cell allow for the 2^(N) independent symbols required formultiplexing N data streams. Accordingly, a 4×4-PPM signal has a cellduration of T (=B⁻¹) and a chip duration of τ=T/4. The electronics andoptical components required for the 4×4-PPM system must be capable of adata rate four times greater than B in order to generate chips with aduration of τ. The 4×4-PPM system has an aggregate data rate of 4B,which is equal to the aggregate data rate of the four input datastreams. The average transmitted power in the m×n-PPM system will beconsiderably less than that in a conventional OOK system, enabling agreater number of DWDM channels without degrading channel bandwidth.

[0019] The factor of improvement in average transmitted power depends onthe amplitudes of the m levels in the m×n-PPM signal relative to theamplitude of the OOK signal. Where the amplitude of the OOK opticalsignal is equal to A_(OOK), then the maximum amplitude (the m^(th)level) of the m×n-PPM optical signal will typically be set in the rangeof A_(OOK) to 2·A_(OOK), depending on the signal-to-noise ratio (SNR)required in the link. The ratio of the average transmitted power of theoptical m×n-PPM signal to that of a corresponding (same aggregate datarate) OOK signal is given by:${\frac{P_{{avg},{PPM}}}{P_{{avg},{OOK}}} = {\frac{A_{PPM}}{A_{OOK}}\quad \left( \frac{m + 1}{mn} \right)}},$

[0020] where A_(PPM) is the maximum amplitude of the m×n-PPM signal andthe assumption is made that chip duration is given by τ=T/n. Returningto the simple case of 4×4-PPM, if A_(PPM)=2·A_(OOK), the averagetransmitted power of the optical 4×4-PPM signal would be a factor of 5/8lower than that of the OOK signal. If A_(PPM)=A_(OOK), then the ratiowould be 5/16. These reductions in average transmitted power can reducenonlinear interactions in the optical fiber, enabling a greater numberof DWDM channels and/or a greater transmission distance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a block diagram of a conventional m-channel WDM fiberoptic transmission system.

[0022]FIG. 2 is a block diagram depicting an exemplary operatingenvironment in which an exemplary embodiment of the present inventioncan be implemented as an encoder.

[0023]FIG. 3 is a graph depicting an exemplary pulse position modulatedsignal over a given time period.

[0024]FIG. 4 is a graph depicting a multilevel pulse position modulatedsignal of an exemplary embodiment of the present invention.

[0025]FIG. 5 is a block diagram depicting a method for generating amultilevel pulse position modulated signal that is an exemplaryembodiment of the present invention.

[0026]FIG. 6 is a block diagram of a transmission system for generatinga multilevel pulse position modulated signal that is an exemplaryembodiment of the present invention.

[0027]FIG. 7 is a block diagram of an encoding circuit for generating amultilevel pulse position modulated signal that is an exemplaryembodiment of the present invention.

[0028]FIG. 8 is a block diagram of a receiving system for receiving anddecoding a multilevel pulse position modulated signal that is anexemplary embodiment of the present invention.

[0029]FIG. 9 is a graph depicting exemplary recovered clock signals thatcan be used by an exemplary receiving system to decode a pulse positionmodulated signal.

[0030]FIG. 10 is a graph depicting a five amplitude-level pulse positionmodulated signal of an exemplary embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0031] In an exemplary embodiment of the present invention, the averagetransmitted power in an optical fiber communication channel is decreasedby using multilevel amplitude modulation in conjunction with PulsePosition Modulation (PPM). This multilevel PPM does not entail anytradeoff between decreased power per channel and channel bandwidth and,therefore, enables a lower average transmitted power compared to On/OffKeying (OOK) with no reduction in aggregate data rate. Accordingly,multilevel PPM can be used in high-speed Dense Wavelength DivisionMultiplexed (DWDM) optical communication systems where the maximumnumber of channels has traditionally been limited by nonlinear effectsassociated with WDM transmission through optical fiber. This modulationtechnique can enable an increased number of channels in DWDM systems,thereby increasing aggregate data rates within those systems.

[0032]FIG. 2 is a block diagram depicting an exemplary operatingenvironment in which an exemplary embodiment of the present inventioncan be implemented as an encoder. Specifically, an exemplary embodimentof the present invention can be implemented as an encoder and/or decoderin an optical fiber communication link. FIG. 2 depicts an exemplarymultilevel ASK optical transmitter 200 that can transmit an opticalsignal over an optical fiber 280 to a multilevel ASK optical receiver250. The transmitter 200 typically receives m input sources 201 andcomprises an error protection coding (EPC) module 210, an m-channelmultilevel PPM encoder 202, which may include a Digital to AnalogConverter DAC (not shown), a pre-compensation or pulse shaping circuit206, and an optical source 208. The combination of the error protectioncoding (EPC) module 210, m-channel encoder 202, andpre-compensation/pulse shaping circuit 206 may be referred to as asymbolizer. The encoder 202 can map an m-bit word (that consists of asingle bit from each of the m input data streams) into an n-bit wordwhere n≧m. The input data can be processed by the EPC module 210 so thatwhen decoded in the receiver, the processed data is error protectedagainst bit errors introduced by the encoding/transmission/decodingprocess.

[0033] Pre-distortion of the transmitted data can help compensate fornon-ideal link frequency response and for some classes of linknon-linearities, effectively reducing pattern-dependent errors in thetransmitted data. Hence, this technique is often referred to aspre-compensation and can be performed by the pre-compensation/pulseshaping module 206. Additionally, the pre-compensation/pulse shapingmodule 206 may perform pulse-shaping to maximize the dispersion distance(i.e., distortion-free transmission distance) of the signal in theoptical fiber 280.

[0034] The receiver 250 typically comprises an optical detector 252, aclock recovery module 254, an n-channel PPM decoder 256, which caninclude an Analog to Digital Converter ADC (not shown), and an errorprotection decoding (EPD) module 258. The combination of the clockrecovery module 254, n-channel decoder 256, and EPD module 258 may bereferred to as a desymbolizer. The electronics of receiver 250 aretermed the “desymbolizer”, because they convert the received symbolsback into one or more digital output data streams. The symbolizer mayalso include post-compensation circuitry (not shown) to further improvethe recovered signal received from the transmitter 200.

[0035] The n-channel PPM decoder 256 converts the received multilevelPPM signal into a stream of n-bit words. The clock recovery circuit 254can be used to generate the necessary timing signal to operate theencoder 256 as well as to provide timing for output synchronization. Then-bit words can be input to the EPD module 258, which converts a codedn-bit word for each clock cycle into the corresponding m-bit word thatwas initially input to the transmitter 200. The original data input tothe transmitter 200 can then be obtained from the EPD 258 by decodingthe error protected data using the redundant bits introduced by thetransmitter's EPC 210 to correct errors in the received data. The EPD258 can output the data in m digital data streams, as the data wasoriginally input to the transmitter 200.

[0036] Compared to on-off keying (OOK), modulation techniques such aspulse position modulation (PPM) can be used to reduce the transmittedpower per channel (i.e., increase power efficiency). However, theincreased efficiency of PPM can simultaneously result in decreasedbandwidth. Using PPM, a transmitted symbol, or cell, is divided into adiscrete number of equally spaced temporal positions. One pulse, orchip, is transmitted per symbol, occupying one and only one of thetemporal positions within that symbol. In this way, data can be encodedinto the temporal position of a chip within its particular symbol.

[0037]FIG. 3 depicts an exemplary 8-PPM (eight temporal positions percell) data stream 300 with a cell period of T and a chip duration of τ,which is one-eighth the cell period. As will be appreciated by thoseskilled in the art, chip duration need not be exactly equal to thespacing of the temporal positions within the symbol. The 8-PPM formatcan be used to multiplex three independent OOK data streams (each withbit rates equal to T-1) since there are eight (23) chip positionsavailable in each symbol. Assuming that each of the three multiplexedchannels consists of a data stream with an equal percentage of 0s and 1stransmitted, the 8-PPM data stream 300 as shown in FIG. 3 will operateat {fraction (1/12)} the average transmitted power of the three OOKchannels combined. Table 1 gives a truth table for multiplexing threeseparate channels into a single 8-PPM data stream. TABLE 1 8-PPM TruthTable Pulse D₁ D₂ D₃ Position 0 0 0 1 1 0 0 2 0 1 0 3 1 1 0 4 0 0 1 5 10 1 6 0 1 1 7 1 1 1 8

[0038] In general, 2^(n) chip positions per cell are required tomultiplex n OOK channels in PPM format. However, in an alternativeembodiment, a guard time interval (not shown in FIG. 3) can be added atthe end of each symbol to contain data that can be used to improvesynchronization at the receiver. The addition of such a time interval toeach symbol requires that chip position spacing within the symbol andchip duration are decreased to make room for the guard time interval.Accordingly, the average transmitted power required for the n-PPM datastream would be a fraction of that required by the n OOK channels. Thisfraction is described by the following equation:$\frac{P_{PPM}}{P_{{OOK},{total}}} = {\frac{1}{n \cdot 2^{n - 1}}.}$

[0039] While PPM advantageously requires less average transmitted powerthan conventional OOK, it can have an adverse side effect: the overallbandwidth in the link can be decreased. In order to multiplex n OOKchannels (each with a bit rate of T⁻¹) in n-PPM format for opticaltransmission, the link would require electronics and optical componentscapable of operating with bit rates of 2^(n)/T. Returning to theexemplary 8-PPM format depicted in FIG. 3, the components required totransmit such a signal would need to be capable of operating at a bitrate of 8/T in order to transmit chips of duration τ=T/8. However, theaggregate data rate of the 8-PPM system would be 3/T (i.e., the numberof channels times the OOK channel bit rate). Accordingly, componentswith a data rate of 2^(n)/T are required to transmit an n-PPM datastream, but the aggregate data rate of the system would only be n/T(i.e., the bandwidth is reduced). For high-speed fiber optic links,there is greater motivation to increase bandwidth than there is todecrease transmission power. Accordingly, PPM is a less thansatisfactory solution for such applications. Various embodiments of thepresent invention can be used to overcome this adverse effect of PPM.

[0040] Although FIGS. 1-3 have been described in the context of n inputstreams (channels), those skilled in the art will appreciate that PPMmay also be used to reduce the average transmitted power on a singlechannel. For example, n consecutive bits in an OOK data stream (with abit rate of B) may be encoded into a 2′-PPM signal with a cell period ofn/B. In this case, the 2^(n)-PPM signal would transmit 1/2^(n−1) theaverage transmitted power of the OOK data stream. However, the 2^(n)-PPMsignal would require components with data rates up to 2^(n)-B/n tomaintain the data rate of the incoming OOK signal. In other words, thetransmitting and receiving components in the link must operate at datarates that are faster than the original data rate, B. As describedabove, a trade-off exists between average transmitted power and thebandwidth of the components.

[0041] Various embodiments of the present invention can decrease theaverage transmitted power per Wavelength Division Multiplexed (WDMchannel), while maintaining bandwidth by implementing multilevelamplitude modulation in conjunction with PPM (i.e., multilevel PPM).Exemplary multilevel PPM methods of the present invention do not requirea tradeoff between decreased power per channel and channel bandwidth,thereby enabling a lower average transmitted power (as compared withOOK) without a commensurate reduction in aggregate data rate. Therefore,multilevel PPM can be implemented in conventional high-speed DWDMsystems where the maximum number of channels is traditionally limited bynonlinear effects such as SPM, XPM, FWM, SBS, and SRS. This modulationtechnique can enable an increased number of channels in DWDM systems,thereby increasing aggregate data rates within those systems.

[0042] In one embodiment of the present invention a method is providedfor combining N data streams into one m-level amplitude modulated n-PPM(n available temporal positions within each transmitted symbol) datastream, where m·n=2^(N). The m levels of amplitude modulation combinedwith the n chip positions within each cell allow for the 2^(N)independent symbols required for multiplexing N data streams. Forclarity, such a modulation format will be denoted as m×n-PPM. As anexample, the simplest case will be considered with N=4 (i.e., four inputdata streams) and m=n=4. FIG. 4 depicts a 4×4-PPM waveform 400 that iscapable of producing 16 (2^(N), N=4) independent symbols, based onunique pulse amplitudes 402 and unique temporal positions 404 within asymbol. It is assumed that each of the 4 independent data streams thatare multiplexed into the waveform shown in FIG. 2 has a bit rate of B(where B=T⁻¹) and that the resulting 4×4-PPM signal has a symbol rate ofB. Thus, the 4×4-PPM signal has a symbol duration of T and a chipduration of τ=T/4. In other words, the electronics and opticalcomponents required for the 4×4-PPM system must be capable of a datarate four times greater than B in order to generate chips with aduration of τ. This 4×4-PPM system has an aggregate data rate of 4B,which is equal to the aggregate data rate of the four input datastreams.

[0043] One advantage of an m×n-PPM transmission system with an aggregatedata rate of B′ is revealed when compared to a single channel OOK systemoperating at the same bit rate of B′. The average transmitted power inthe m×n-PPM system will be considerably less than that in the OOKsystem, enabling a greater number of DWDM channels without a degradationof channel bandwidth. The factor of improvement in average transmittedpower depends on the amplitudes of the m levels in the m×n-PPM signal ascompared to the amplitude of the OOK signal. If the amplitude of the OOKoptical signal is equal to A_(OOK), then the maximum amplitude (them^(th) level) of the m×n-PPM optical signal will typically be set in therange of A_(OOK) to 2·A_(OOK), depending on the signal-to-noise ratio(SNR) required in the link. The ratio of the average transmitted powerof the optical m×n-PPM signal to that of a corresponding (same aggregatedata rate) OOK signal is given by:${\frac{P_{{avg},{PPM}}}{P_{{avg},{OOK}}} = {\frac{A_{PPM}}{A_{OOK}}\quad \left( \frac{m + 1}{mn} \right)}},$

[0044] where A_(PPM) is the maximum amplitude of the m×n-PPM signal andthe assumption is made that chip duration is given by τ=T/n. Returningto the simple case of 4×4-PPM, if A_(PPM)=2A_(OOK), the averagetransmitted power of the optical 4×4-PPM signal would be a factor of 5/8lower than that of the OOK signal. If A_(PPM)=A_(OOK), then the ratiowould be 5/16. These reductions in average transmitted power can reducenonlinear interactions in the optical fiber enabling either a greaternumber of DWDM channels or a greater transmission distance.

[0045]FIG. 5 depicts an exemplary method for generating a multilevel4×4-PPM signal 500. Two OOK data streams 502, 504 are combined into amultilevel signal 506, while two other OOK data streams 508, 510 arerepresented as chip positions in a 4-PPM signal 512. Next, the amplitudelevels of the multilevel signal 506 are used to determine the amplitudeof each transmitted chip in the PPM signal 512. In this manner, a4×4-PPM signal 500 can be generated and used, for example, to drive anoptical transmitter. It should be clear to those skilled in the art thatmore data streams 502, 504, 508, 510 (which will require more levels inthe multilevel signal, more chip positions in the PPM cell, or both) maybe multiplexed in this manner for even greater channel efficiency.

[0046]FIG. 6 depicts a transmission system 600 for generating amultilevel PPM signal. Four independent OOK electrical data streamsD₁-D₄ are multiplexed into a single 4×4-PPM electrical signal 610 thatcan then be used to modulate an optical transmitter (not shown). In thisexample, each of the four OOK signals D₁-D₄ has the same bit rate, B.Two of the binary signals, D₁ and D₂, are combined by an adding element604 to form a multilevel signal 602 with a symbol rate equal to B. Priorto summation, D₂ is attenuated by −6 dB by an attenuator 606, and D₁ isdelayed by an amount equivalent to the delay applied to D₂ by theattenuator 606. The multilevel signal 602 is then sent to a multilevelPPM encoder circuit 08. Those skilled in the art will appreciate that amore sophisticated multilevel encoding scheme may be used to combineincoming data channels into a single multilevel amplitude waveform inorder to maximize the SNR of the multilevel signal and enhance receiversensitivity.

[0047] Referring still to FIG. 6, the signals D₃ and D₄ are also inputto the encoder circuit 608 after each is appropriately delayed to ensureproper synchronization with the multilevel signal 602. The encodercircuit 608 generates a 4-PPM chip sequence based on the current bitcombination of the signals D₃ and D₄. The amplitude level of eachgenerated chip in the 4-PPM signal is set by the amplitude of themultilevel signal (plus a small DC component). The resulting signal is a4×4-PPM waveform that can be used to modulate an optical transmitter. Asa specific example, the four OOK signals D₁-D₄ can be assumed to eachhave bit rates of 2.5 Gb/s. The multilevel signal generated at theadding element 604 will then have a symbol rate of 2.5 Gsymbols/s. Thetransmitted 4×4-PPM signal would have a symbol rate of 2.5 Gcells/s andfour temporal chip positions per cell. Thus, the electronic circuit ofthe encoder 608 and any downstream optical transmission components(modulator, laser, etc.) must be capable of operating at a data rate of10 Gb/s. The aggregate data rate of the 4×4-PPM system would be 10 Gb/s.As described in more detail above, the average transmitted power of the10 Gb/s 4×4-PPM system would be much less than that of a conventional 10Gb/s OOK system.

[0048] The implementation depicted in FIG. 6 is described as anexemplary embodiment of the multilevel PPM of the present invention. Itwill be appreciated by those skilled in the art that more than twosignals may be combined into a multilevel signal at the adding element604 and that multiple data streams may be combined using well-knownefficient coding techniques as opposed to a simple adding element.Likewise, more than two OOK signals could be input to the multilevel PPMencoder, requiring greater than four temporal positions per cell.However, as the number of input OOK data streams increases so will thecomplexity of the electronics, since a greater number of inputs willrequire faster components to avoid bottlenecking.

[0049]FIG. 7 depicts an exemplary embodiment of a multilevel PPM encodercircuit 700 for 4×4-PPM transmission. The inputs signals to the circuitinclude a multilevel signal that maintains greater than 1 bit per symbol(in this case, D₁+D₂), two independent data channels (D₃ and D₄), aclock signal 702, and a “clock x 2” signal 704. The clock 702 and clock×2 704 are input to an AND gate 706 in order to generate one pulse perbit with a temporal width equal to one chip duration, τ. The generatedpulse can be represented as a chip in the first position of a PPM signalwith a cell period equal to the bit period of the incoming data signalsD₁-D₄. This timing pulse is used as an ENABLE signal for a 2-to-4decoder 708. The data signals D₃ and D₄ are input to the decoder 708.When the decoder 708 is enabled by the timing pulse, it transmits an“on” state to one of four output channels according to the truth tableshown in Table 2. TABLE 2 Decoder Input Decoder Output ENABLE D₃ D₄Channel 0 Channel 1 Channel 2 Channel 3 0 X X 0 0 0 0 1 0 0 1 0 0 0 1 10 0 1 0 0 1 0 1 0 0 1 0 1 1 1 0 0 0 1

[0050] Because the decoder 708 is enabled only for a duration, τ, foreach bit, the output “on” state is also a pulse of duration τ. Eachoutput channel from the decoder acts as an ENABLE for a transmissiongate (TG) 710-716. Each of the four transmission gates 710-716 isconnected in parallel to the incoming multilevel signal (D₁+D₂). Thus,when a transmitted pulse from the decoder 708 enables its associatedtransmission gate 710-716, the gate will transmit a pulse with anamplitude equal to that of a current bit from the multilevel signal anda pulse width equal to τ.

[0051] The multilevel signal may be generated with a DC offset at itslowest level (not shown in the figure) since a zero (0) amplitude may beindeterminable in a PPM format. The output of each transmission gate710-716 has a corresponding delay 718-722 associated with it thateffectively assigns the appropriate chip position (in a cell) to eachtransmitted multilevel pulse. The four channels are combined at anadding element 724 and the resulting waveform is a 4×4-PPM signal 726that can be used to drive an optical transmitter. Those skilled in theart will appreciate than an m×n-PPM signal can be generated by usingfaster clock rates (for n chip positions) and a multilevel signal with mlevels.

[0052] FIGS. 5-7 have been primarily devoted to the description of thetransmission of a multilevel PPM signal, as opposed to the reception ofa multilevel PPM signal. FIG. 8 depicts an exemplary embodiment of amultilevel PPM receiver 800. Specifically the receiver 800 of FIG. 8could be implemented in connection with a 4×4-PPM modulation scheme.Those skilled in the art will appreciate that various well-known clockrecovery methods, such as a phase-locked loop (PLL) method, can be usedto generate a clock 802 at four times the cell rate from the received4×4-PPM signal. The clock can then be frequency divided by four toproduce a clock at the cell rate. The clock recovery step is implied inFIG. 8. For the receiver 800 of FIG. 8, the clock rate is equal to thatof the received 4×4-PPM cell rate. For example, if the 4×4-PPM signalhas a cell rate of 2.5 Gcell/s, the recovered and subsequently frequencydivided clock 802 would have a rate of 2.5 GHz. The recovered clock 802depicted in FIG. 8, is input to a phase generator 804 which generatesfour clock signals (φ1, φ2, φ3, and φ4) with the same rate as the inputclock signal (not shown), each with a π/4 phase shift relative to itsnearest neighbor. Exemplary clock signals (φ1-φ4) are illustrated inFIG. 9. Those skilled in the art will appreciate that the phasegenerator 804 can be a power splitter with four different delay lines onits four outputs. Alternatively, the phase generator 804 could beincorporated into the clock recovery circuit.

[0053] The 4×4-PPM signal (labeled Vin) in FIG. 8, is input to fourseparate 5-level (or log2(5)=2.322-bit) analog-to-digital converters(ADCs) 806-812. Each ADC 806-812 has three outputs, D1, D2, and Ref.Each ADC 806-812 is triggered to make a decision by the leading edge ofone of the four clock signals that are output from the phase generator804. The relative phase shifts of the clock signals ensure that, foreach received cell, each of the 4 chip positions in the 4×4-PPM cell areevaluated by one of the ADCs 806-812 for a multilevelintensity-modulated pulse. The outputs of each ADC are input to adecoder circuit 814, depicted in FIG. 8. The 2.322-bit ADCs 806-812 inFIG. 8 can operate with four decision thresholds to accommodate the factthat the received 4×4-PPM signal can have 5 possible amplitudes. Aconventional 2-bit ADC operates with three decision thresholds for usewith a 4-level amplitude modulated signal.

[0054]FIG. 10 depicts an exemplary 4×4-PPM signal 1000, where Levels 1-4correspond to a 4-level amplitude modulated signal, while Level 0signifies an empty chip position within the PPM cell. Accordingly, if atriggered ADC receives Levels 1, 2, 3, or 4, the ADC will output twoparallel bits based on the input amplitude level on the outputs D1 andD2 (FIG. 8). However, if the triggered ADC receives Level 0, the ADCwill output a special bit on the Ref (FIG. 8) output which signifies tothe decoder 814 that D1 and D2 (FIG. 8) do not carry any information.

[0055] For each cell cycle, the decoder circuit 814 uses the four Refoutputs from the ADCs 806-812 to determine both the chip position forthe current cell as well as which of the ADCs is outputting the“amplitude-encoded” data on channels D1 and D2. Those skilled in the artwill appreciate that these decoding functions can be accomplished usingvarious well-known combinational logic circuits. For each cell cycle,the decoder 814 outputs four bits (two “amplitude-encoded” bits from theappropriate D1 and D2 outputs and two “position-encoded” bits) to alatch 816 as depicted in FIG. 8. As the 4×4-PPM signal is transmittedthrough an optical communication system, the individual pulses maybroaden in width, causing the ADCs to incorrectly output data on D1 andD2. Those skilled in the art will appreciate that the Decoder 814 can bedesigned to effectively select the strongest input signals, therebyreducing or eliminating errors that may result from chips broadeninginto neighboring positions. The latch 816 is triggered by the originalrecovered and frequency divided clock signal 802. The latch 816 outputsin parallel four bits per PPM cell cycle. Depending on the requirementsof the system in which this 4×4-PPM receiver 800 is implemented, thesebits may then be multiplexed to a single data stream that is four timesthe data rate of the four parallel streams.

[0056] The use of multilevel amplitude modulation can result in asignal-to-noise (SNR) penalty when compared with OOK modulation at anidentical base symbol rate. Because this SNR penalty may manifest itselfin multilevel PPM modulation as well, various embodiments of the presentinvention can include means for reducing an SNR penalty. Conventionalmethods for reducing or eliminating an SNR penalty include the use ofForward Error Correcting (FEC) codes as well as a combination ofprecompensation techniques and pulse-shaping techniques. Anothertechnique applicable to various embodiments of the present invention canbe implemented using the Decoder (814) of FIG. 8. Because a decoder canbe used to effectively select (i.e., “vote” for) the strongest inputsignal, this “voting” mechanism in the decoder can be configured toenhance the effective sensitivity of the receiver. This wouldeffectively reduce the SNR penalty associated with multilevel amplitudemodulation. The inventors contemplate that those skilled in the art willappreciate that various techniques are well-known and available forovercoming an SNR penalty, any of which might be applicable to thereduction of an SNR penalty in the multilevel PPM modulation context.

[0057] Although the present invention has been described in connectionwith various exemplary embodiments, those of ordinary skill in the artwill understand that many modifications can be made thereto within thescope of the claims that follow. Accordingly, it is not intended thatthe scope of the invention in any way be limited by the abovedescription, but instead be determined entirely by reference to theclaims that follow.

What is claimed is:
 1. A method for decreasing the transmitted power ina channel of an optical fiber communication system while maintaining achannel bandwidth, the method comprising the steps of: receiving adigital input signal, comprising a series of input pulses, each inputpulse having one of two pulse levels; creating a digital input wordhaving n bits from the digital input signal; and converting each digitalinput word to a corresponding output symbol representing one of 2^(n)distinct values; wherein the output symbol comprises a multilevel pulseposition modulated symbol.
 2. The method of claim 1, wherein the outputsymbol is characterized by representing one of a plurality of uniqueamplitudes.
 3. The method of claim 1, wherein the output symbol ischaracterized by representing one of a plurality of unique chippositions.
 4. The method of claim 1, further comprising the step ofgenerating an output signal comprising a series of output symbols. 5.The method of claim 4, wherein the output signal has an aggregate datarate equal to a data rate of the input signal.
 6. The method of claim 5,wherein the output signal has a lower transmitted power than an on-offmodulated signal transmitting a series of on-off modulated outputsymbols at the aggregate data rate.
 7. The method of claim 6, whereinthe transmitted power of the output signal is less than the transmittedpower of the input signal by a factor of at least ½.
 8. An optical fibercommunication system comprising: a multilevel pulse position modulationtransmitter for combining n input signals for transmission over anoptical fiber communication link as an encoded output signal; whereinthe encoded output signal comprises a series of multilevel pulseposition modulated symbols, each symbol representing one of 2^(n) uniquevalues.
 9. The optical fiber communication system of claim 8, wherein afirst transmission power required to transmit the encoded signal overthe optical fiber communication link at a predetermined data rate isless than a second transmission power required to transmit an on-offmodulated output signal over the optical fiber communication link at thepredetermined data rate.
 10. The optical fiber communication system ofclaim 9, wherein the first transmission power (P_(FIRST)) required tomultiplex n on-off keyed input signals is a fraction of the secondtransmission power (P_(SECOND)) and is given by:$\frac{P_{FIRST}}{P_{SECOND}} = {\frac{1}{n \cdot 2^{n - 1}}.}$


11. The optical fiber communication system of claim 8, furthercomprising a receiver operative to receive the encoded signal from theoptical fiber communication link and having a decoder operative todecode the encoded output signal into n output signals.
 12. The opticalfiber communication system of claim 11, wherein the n output signals aresubstantially identical to the n input signals.
 13. A method forencoding n on-off keyed input signals into a single multilevel pulseposition modulated output signal, the method comprising the steps of:combining at least two of the n input signals into a multilevel signal;representing at least two other of the n input signals as chip positionsin a four position pulse position modulation signal; and modifying atleast one amplitude level of the four position pulse position modulationsignal in accordance with at least one amplitude level of the multilevelsignal to generate the single multilevel pulse position modulated outputsignal.
 14. The method of claim 13, wherein the step of combining atleast two of the n input signals into a multilevel signal comprises thesteps of: attenuating a first signal of the at least two of the n inputsignals; and adding the first signal to a second signal of the at leasttwo of the n input signals.
 15. The method of claim 14, wherein thefirst signal is attenuated by 6 dB.
 16. The method of claim 14, furthercomprising the step of delaying the second signal for a first delaytime, wherein the first delay time is equal to a second delay timeimposed on the first signal by the attenuating step.
 17. The method ofclaim 14, further comprising the step of delaying the at least two otherof the n input signals for a third delay time, wherein the third delaytime is equal to a fourth delay time imposed on the at least two of then input signals by the attenuating step and the delaying the secondsignal step.
 18. A transmitter for use in an optical fibercommunications system, comprising: a multilevel pulse positionmodulation encoding circuit operative to encode at least four on-offkeyed input signals into a single multilevel pulse position modulatedoutput signal; a pulse shaping module for processing the output signalto maximize a dispersion distance of the output signal; and an opticalsource for transmitting the output signal over a link of the opticalfiber communications system.
 19. The transmitter of claim 18, whereinthe encoding circuit is operative to combine at least two of the on-offkeyed input signals into a multilevel signal.
 20. The transmitter ofclaim 19, wherein the encoding circuit is further operative to representat least two other of the on-off keyed input signals as chip positionsin a four position pulse position modulation signal.
 21. The transmitterof claim 20, wherein the encoding circuit is further operative to modifyat least one amplitude level of the four position pulse positionmodulation signal in accordance with at least one amplitude level of themultilevel signal.